1. Field of the Invention
The present invention generally relates to a pulse generating circuit for self-refresh, and more specifically, to a pulse generating circuit for self-refresh which regulates a pulse generation cycle by regulating the amount of charges by using a capacitor, thereby evaluating reliability.
2. Description of the Prior Art
In mobile or portable apparatus such as a cellar phone or a lap-top computer, it is important to embody low power function. Specifically, it is important to reduce the amount of current required in a self-refresh mode in order to embody the low power function in a DRAM.
In order to reduce the amount of current required in the self-refresh mode, operations are provided such as a Partial Array Self Refresh (hereinafter, referred to as “PASR”), a Temperature Compensated Self Refresh (hereinafter, referred to “TCSR”), and a Deep Power Down (hereinafter, referred to as “DPD”) mode. Of these methods, a user can program the PASR and the TCSR with an Extended Mode Register Set (hereinafter, referred to as “EMRS”).
Generally, data retention time of the DRAM is shortened as temperature more increases. As a result, a self-refresh cycle of the programmed TCSR is changed depending on temperature set by a user. Specifically, when the TCSR is used at low temperature, the self-refresh cycle is set to be long so that the amount of current can be reduced.
However, when the usage temperature of the DRAM is beyond the set range in the programmed TCSR, the reliability of the DRAM operation cannot be secured. As a result, the conventional EMRS-TCSR is required to be used restrictively.
In order to solve the above-described problems, an auto TCSR has been suggested. In the auto TCSR, temperature is not set by a user but is sensed in a chip, and a generation cycle of a refresh signal TEMPOSC (Temperature Oscillation) is automatically regulated depending on the sensed temperature.
Specifically, an auto TCSR having a temperature sensor in a memory chip is called on Die TCSR. The auto TCSR lengthen the refresh cycle at low temperature by using the amount of current differentiated in a diode depending on temperature.
FIG. 1 is a diagram of a conventional pulse generating circuit for self-refresh with a diode.
The pulse generating circuit of FIG. 1 comprises a voltage comparator 10, a delay circuit 12, a control unit 14 and a temperature sensor 16. The voltage comparator 10 comprises a differential amplifier 20 and a reference voltage supply unit 22. The delay circuit 12 comprises a chain of an inverter and a capacitor which are connected in parallel. The control unit 14 is formed of combination of a NAND gate and an inverter.
The differential amplifier 20 compares a voltage, which is fed-back from the temperature sensor 16 and applied to a capacitor C1, with a voltage, which is supplied from the reference voltage supply unit 22 and applied to a capacitor C2, and then outputs the comparison result to the delay circuit 12. Here, the voltage applied to the capacitor C1 is charged as diode current is fed back through a node A.
The delay circuit 12 is used to secure charge and discharge time of the capacitor C1 in the differential amplifier 20.
The control unit 14 outputs a pulse where temperature is sensed in response to a control signal TEMPON or outputs a high level voltage.
The temperature sensor 16 is a circuit with a diode, and current flowing in a MOS diode has a temperature function. That is, when a gate source voltage Vgs is below about 3V, the amount of current is reduced as temperature becomes lower.
In other words, charges in the capacitor of the delay circuit 12 are discharged through the diode of the temperature sensor 16. Here, when the charges are discharged over at a predetermined level, a pulse is generated by the voltage comparator 10. Here, sensing diodes D1, D2 and D3 are connected in parallel for setting temperature. The sensing operation is performed depending on temperature setting by selectively operating switching transistors T1, T2 and T3. The current determined by the sensing diodes D1, D2 and D3 is transmitted to the voltage comparator 10 through the node A.
The above-described pulse generating circuit generates a pulse by charge through the capacitor and discharge through a diode. The generation cycle of the pulse can be regulated corresponding to the temperature depending on selection of the diode.
However, the amount of current flowing in the diode is changed sensitively depending on temperature. Therefore, the above-described pulse generating circuit for self-refresh which senses temperature with a diode has large cycle distribution even in the same lot or the same wafer.
When the pulse generating circuit actually performs an operation to compensate temperature as a diode, the relation between the self-refresh cycle (TEMPOSC cycle) and refresh current IDD6 which are measured in the same wafer can be measured as shown in FIG. 2. That is, the self-refresh cycle has a difference of more than two times depending on the refresh current in the same wafer.
As shown in FIG. 2, a cycle of the refresh signal TEMPOSC is required to be adjusted to regulate the refresh current IDD6 having the large distribution below at a predetermined level.
A conventional refresh circuit regulates the cycle of the refresh signal TEMPOSC in comparison with a basic cycle. As a result, refresh fail can increase by the operation characteristic of the diode sensitive to temperature.
For example, if the refresh signal TEMPOSC of the Die having a basic cycle as λ(μs) at 85° C., the refresh time by the refresh signal TEMPOSC becomes 32 ms in case of 4 division and 4 k cycle refresh. Here, if the refresh time is set to be 64 ms in order to reduce the current IDD6, the division is 8 or the cycle is 4.0 μs. However, the above-described case may cause the following problem.
When temperature which is 8 times of the basic cycle (normal self-refresh) is T, the refresh signal TEMPOSC having 4 division around the temperature T has a refresh time of (2+α)*4*4 k. However, if the refresh signal is trimmed at 8 division, the refresh time is (2+α)*8*4 k so that it increases two times in comparison with 4 division.
When the cycle of the refresh signal TEMPOSC is 2.0 μs around the temperature T, the cycle of the refresh signal TEMPOSC is 2+α=8*λ. However, if the cycle increases two times to 4.0 μs, the cycle of the refresh signal TEMPOSC is 2* (2+α) around the temperature T. As a result, the cycle is 8 times larger than the basic cycle so that the refresh signal TEMPOSC is reset at a temperature higher than the temperature T.
As described above, in the conventional pulse generating circuit to compensate temperature with a diode, self-refresh current is changed sensitively to temperature, and the difference of the cycle distribution becomes larger depending on object even in the same lot or the same wafer.
Therefore, it is difficult to test reliability of the pulse operation for self-refresh under a predetermined condition.